This invention relates to a fluorescent display device having a low energy electron excitation phosphor and more particularly pertains to such device in which SnO.sub.2 :Eu is employed as a phosphor for emitting red fluorescent rays.
A fluorescent display device having a low energy electron excitation phosphor basically has the structure of a modified triode vacuum tube, in which the phosphor coated on a anode plate fluoresces due to bombardment of electrons emitted from a cathode heated at subluminous temperature. The electron bombardment is controlled by a grid.
Such device has the following features: Its power dissipation is quite low, being about one-half to two-thirds of a LED and it has a simpler and more easily producible structure than the LED. Further it is superior to a liquid-crystal device because of self-glowing characteristics which makes the device visible in a dark place.
Therefore, the vacuum fluorescent device has flourished as a dot-matrix alphanumeric device which is assembled in, for example desktop calculators.
As the low energy electron excitation phosphor, only ZnO:Zn has been popular. It emits blue-green rays. However for enabling more complex displays, it has been desired to make available another-color-ray emitting phosphor.
SnO.sub.2 :Eu phosphor was invented for such purpose and emits red rays having the spectrum shown in FIG. 1 when excited by electron beams or ultraviolet rays. This phosphor exhibits a dead voltage of 5 V which is as low as that of ZnO:Zn phosphor. Thus SnO.sub.2 :Eu phosphor is the first red ray emitting and low energy electron excitation phosphor which is satisfactory in performance and reliability for commercial use.
The SnO.sub.2 :Eu phosphor has hitherto been produced by the following process:
Mixed aqueous solution of stannous chloride and europium chloride is prepared and ammonia solution is added thereto until pH of the mixed solution becomes to 8.5, thereby forming hydroxide coprecipitate. By drying and heat-treating the coprecipitate, SnO.sub.2 :Eu phosphor is obtained. This hydroxide coprecipitate, however, has very small particle size of 0.01 .mu.m and because of its large apparent specific volume due to such extremely small particle size, it is difficult to handle in the manufacturing process. This coprecipitate has a further disadvantage that it easily changes to SnO.sub.2 in the drying step, resulting in the decrease of reaction activity in the following heat treatment. As the result of those disadvantages, the obtained phosphor exhibits unsatisfactory performance for the commercial use of the red emission fluorescent vacuum display device.
Another method is as follows:
Oxalate coprecipitate is made by rapidly mixing stannous chloride and europium chloride aqueous or alcohol mixed solution, with the aqueous solution including oxalic ions. The phosphor is obtained as the result of thermal decomposition of the coprecipitate. The oxalate coprecipitate has a dendrite-like particle configuration and thus, the resulting phosphor hardly has particle size more than 1.5 .mu.m and does not emit a glow of satisfactory intensity. Further such oxalate coprecipitate has a disadvantage in that it thermally decomposes so rapidly during the heat treatment that there are yielded CO and CO.sub.2 gases, resulting in the self-spouting phenomenon from a container.